If most of the lights are ~3200K (halogen, tungsten), I'd put a Full CTO filter over the flash, and set the camera's WB to Tungsten. That'll get them in the ballpark, and shooting raw will let you tweak it later.
That said, I've shot a lot of portraits with a single flash shot through a medium white umbrella. Works well for a small group, too. Adding a big piece of white foamcore on the opposite side, or shooting next to a white wall, will open up the shadows. And the background just does whatever it wants. Rim lights are often more trouble than they are worth for multiple subjects.
Often I'll drag the shutter a bit to get the background to open up, but you have to be careful about it -- window light behind them is good, fluorescent not so good.